1. Field of the Invention
The present invention relates to a cleaning unit for use in an image-formation apparatus such as an electrophotographic copying machine, laser printer, facsimile machine, and the like, and more particularly to a cleaning unit capable of preventing the occurrence of image flow.
2. Discussion of Background
In an image-formation apparatus such as an electrophotographic copying machine, laser printer, facsimile machine, and the like, there are commonly used photoconductors comprising an electroconductive support and a photoconductive layer formed thereon comprising an inorganic photoconductive material such as an amorphous silicon (a-Si photoconductors), Se, As2Se3, SeTe and the like (Se photoconductors), or an organic photoconductive material such as poly-N-vinylcarbazole, trinitrofluorenone, and various types of azo dyes and the like (OPC photoconductors).
For example, an OPC photoconductor exhibits good electrical characteristics and spectral sensitivity, has a low manufacturing cost, is non-polluting, and can be formed into a photoconductor with a belt or drum shape with relative ease. It therefore finds many applications ranging from low-speed to medium-speed machines.
An a-Si photoconductor has an inferior charging performance in comparison with other photoconductors, but it has high sensitivity and excellent resistance to wear, so is utilized, for instance, in high-speed copying machines, and laser printers.
One example of an image-formation apparatus which uses this type of photoconductor is shown in FIG. 10. A photoconductor 1 is uniformly charged by a corona discharger 2. The corona discharger 2 may be a corotron type on which a 40 .mu.m to 100 .mu.m diameter tungsten wire is mounted, or a scorotron type in which, in addition, a grid is provided in the vicinity of an open section of a corona discharger in order to make the nonuniform discharges uniform, and a high voltage of 4000 to 8000 volts is applied. After a latent electrostatic image is formed on the photoconductor 1 in an exposure section 3, the latent electrostatic image is developed to a visible toner image by a development unit 4.
The toner image is transferred to a copy paper 9 by an image transfer corona charger 5, then the copy paper 9 is separated from the photoconductor 1 by a sheet-separation corona charger 6, is fixed to the copy paper 9 by an image-fixing unit 10, and turned into a hard copy. The toner image on the photoconductor 1 is cleaned after the image transfer by a cleaning unit 7 to complete a cycle of the copying process.
However, it is known that when the corona discharge takes place in the image-formation apparatus using the corona discharger, corona products, such as ozone and nitrogen oxides are produced, and when these corona products adhere to the surface of the photoconductor, the surface resistivity of the surface layer of the photoconductor is lowered and both the photosensitive characteristics of the photoconductor and the image quality characteristics deteriorate. In particular, the surface resistivity drops depending on the humidity and a blurred image is produced. In the worst case there is complete failure in the formation of the image. Accordingly, in order to maintain the initial image quality over a long period it is necessary to eliminate the effects of the corona products. In this type of blurred image, the degree of formation varies according to the material used to form the photoconductor, and in addition, there are differences in the materials which cause the image blurring to develop, but in all cases it is the corona products that trigger the development of the blurring.
The following factors are known to prevent the deterioration of the image characteristics caused by these types of corona products. As a first example, an improvement of the material itself from which the photoconductor is constructed is known to prevent a drop in the resistivity of the surface. In detail, this involves a material which forms a photoconductive layer on an electroconductive support member in the photoconductor, and a material further laminated on this photoconductive layer as a protective layer. In the case where the photoconductive layer and the protective layer are formed by a spray method or a coating method, an antioxidant, such as amine type or hydroxylamine type antioxidant, is added or rubbed in from the outside, to remove the effect of the corona products.
As a second example, it is known that the effect of ozone is eliminated by improving the corona charger itself, thereby restraining the production of the corona products, or by preventing the corona products from depositing on the photoconductor.
In the former, the charge wire and shielding casing or the grid are plated with a metal such as Au, Ag, Pt, Pd, Ni and Fe, or a metallic oxide such as Ni.sub.2 O.sub.3, BaO, alumina, or chromium oxide, which serves as an agent for inhibiting the generation of ozone, whereby the development of corona products during the corona discharge process is restrained, as disclosed in Japanese Laid-Open Patent Applications 64-68774, 47-37547, 49-40739, and 49-84660,
In the latter, for example, as disclosed in Japanese Laid-Open Patent Application 63-311365, the inner wall of the shielding casing or the grid is treated with activated carbon fiber or manganese oxide, or a metal chelate compound, and the corona products are absorbed to prevent their deposition on the photoconductor. In addition, other methods include forming the grid from an activated carbon fiber system, or attaching an absorption member, for example, as disclosed in Japanese Laid-Open Patent Application 1-210974, or adapting the shape of the shielding casing to take wind flow into account. Further, there is also the combination of plating the shielding casing with Pt or Ag and an absorption agent made from activated carbon, for example, as disclosed in Japanese Laid-Open Patent Applications 50-34828 and 52-133894.
As a third example, heating the photoconductor by a heater or drying with hot air to remove the effect of moisture, and use of a substance to prevent a drop in the resistivity of the surface of the photoconductor are also known, for example, as disclosed in Japanese Laid-Open Patent Applications 59-208558, 60-95467, 61-132977, and 62-262065.
As a fourth example, there is a method by which the corona products adhering to the surface of the photoconductor are physically removed by scouring or by wet cleaning. A steel wire wrapped around a roller or blade is used in the scouring method, as disclosed, for example, in Japanese Laid-Open Patent Application 1-161281, while, in the wet method, water or a solution is used to remove the corona products from the surface of the photoconductor.
Also, in addition to the fourth example, other methods for preventing image deterioration caused by the corona products are known, for example, as disclosed in Japanese Laid-Open Patent Applications 58-28581, 60-95459, 60-189769, 60-102659, 59-219770, 60-134254, 60-17765, and 55-155369.
In an image-formation apparatus such as that illustrated in FIG. 10, a corona discharge apparatus is used to perform the charging of the photoconductor, image transfer, and transfer-sheet separating operations, but corona products such as zone (O.sub.3), and nitrogen oxides (NO.sub.x), are produced by the corona discharger during corona discharge. As a result, these corona products are changed into nitrogen compounds or hydrophilic compounds including aldehyde group and/or carboxyl group, from the action of the discharge energy and of the moisture, carbon dioxide gas, and nitrogen gas in the air, so that the surface of the photoconductor is oxidized. Furthermore, the electric surface resistivity of the photoconductor is decreased because of the absorption of these compounds or moisture in the air so that the image flows or the copy quality is severely reduced, with widespread loss of the image because of the phenomenon by which blank spot-shaped sections appear.
This phenomenon has a strong influence in the case where an alternating current or a negative voltage is applied to the corona discharger. There are two types of phenomena which cause the image to disappear; a belt-shaped image loss produced under the corona discharger when the image-formation apparatus halts; and an image flow over the entire surface, which occurs in a highly humid atmosphere of 80-90% RH. These phenomena are produced more or less in most photoconductors, but, in the case of a photoconductor using a photoconductive layer made of an a-Si, a hydrophilic material such as SiO.sub.2 is produced on the surface of the photoconductive layer, so that there is a tendency toward image flow. Also, the same type of problem occurs with a photoconductor using a protective layer of a - C : H, which is considered to be superior in not only durability but also weather resistivity, as with an a - Si photoconductor.
With this type of photoconductor, the severity of the image flow increases as the humidity increases and can extend over the entire image.
When an As2Se3 photoconductor is used alone, the problem of image flow does not occur, but if an overcoat layer of an organic resin such as an ester or an urethane crosslinked-type of styrene - methyl methacrylate resin in which ultrafine particles of SnO.sub.2, SnO.sub.2 /Sb.sub.2 O.sub.3 or TiO.sub.2 are dispersed is used, it is known that a severe image flow occurs. Such an image-flow-producing process varies according to the structure of the photoconductor, but in all cases the existence of the corona products is the cause.
This phenomenon is not produced when the photoconductor is new, but after repeated use of the photoconductor in an image-formation apparatus, this phenomenon conspicuously occurs. This is because after repeated use of the photoconductor, the surface becomes soiled and the water repellency thereof decreases, so that the absorptivity of the dirt is increased and hydrophilic materials tend to remain on the surface. Most of the dirt adhering to the photoconductor cannot be removed by a cleaning blade or a simple cleaning means so the effect of the dirt remains for a long time.